Comstock/Thinkstock(HOUSTON) -- The fiance of a 21-year-old Houston woman killed in a police car crash while in custody is demanding to know why officers didn't restrain her with a seatbelt.

Andrew Durham was waiting up for three and a half hours for his fiancée, Caroline Nicolai, to come home in the early hours of Wednesday morning, but she never appeared.

"I want to know what the officer was thinking and why she didn't restrain her properly knowing that it's 3 a.m. and there's people driving around drunk," Durham told ABC Houston affiliate KTRK-TV. "And I want to know what they are going to do about that, because I'm sorry, the 'Oh, you messed up; don't do it again,' is not acceptable in my book."

Nicolai was thrown from the back of a police vehicle when another car driven by an alleged drunken driver ran a red light at an intersection and smashed into the patrol car from behind.

Police were taking an intoxicated Nicolai into custody for her own protection, after finding her wandering around Hobby Airport. They were en route to a sobering center approximately 3:35 am when the red Chevrolet Cobalt plowed into them, striking the back left passenger door of the police vehicle, according to police.

Preliminary police investigations show Nicolai was handcuffed but not wearing a seatbelt at the time -- a violation of Houston Police Department procedures. Police say she was transported from the scene of the incident to Ben Taub General Hospital, where she was pronounced dead.

Houston police would not comment on how far Nicolai was thrown from the police car or the exact injuries she suffered that led to her death.

"That's all part of the investigation -- that particular incident is part of an internal affairs investigation," police spokesman Victor Senties told ABC News.

The victim's parents did not immediately respond to calls from ABC News.

The arresting officer, Regina Gonzales, who was driving the police car, was taken to Memorial Hermann Hospital with minor injuries. She remains on duty and was not available for an interview with ABC News.

The driver of the other Chevy, Nicole Moser, 21, who has since been charged with intoxication manslaughter, admitted to police she was leaving a bar with a fellow unidentified passenger at the time, authorities said.

"The female driver of the Cobalt, she admitted to drinking alcoholic beverages. She pretty much did not know what had happened," Sgt. Izaguirre told reporters at the scene. "She stated to one of our officers here that she should not have been driving. She should not have been driving."

A devastated Durham posted an angry message to YouTube Wednesday admonishing the officer who arrested Nicolai. In the clip titled "Police negligence killed my best friend and love," Durham tells of his fiancée's plans to go to school for the culinary arts before her life was tragically cut short.

"The drunk driver got manslaughter charges. They're going to stick," says Durham. "I believe the police officer needs to get the same thing, because due to her negligence, she cost the world a beautiful, smart, intelligent 21-year-old girl who still had her entire life before her."